We intend to isolate and characterize mutants of Salmonella typhimurium deficient in the enzymatic modifications of tRNA. One tRNA methylase mutant has been recently isolated and its phenotype has suggested two ways of selecting for more tRNA modification deficient mutants, i.e., isolating recessive suppressor and adenine sensitive strains. Certain regulatory mutants in bacteria have also proved to be deficient in tRNA modification suggesting another method of selection for these mutants. Until now the difficulty in studying specifically unmodified tRNA has been in obtaining tRNA lacking only one minor nucleoside. With these selection techniques we hope to isolate several bacterial strains with specifically undermodified tRNA. These strains will then be examined in order to see what effects the undermodified tRNA has on cell physiology. The tRNA and the defective modifying enzyme will also be characterized in vitro. In correlating the in vitro properties of undermodified tRNA with the effects this tRNA has on cell physiology, we hope to understand the biological significance of tRNA modifications.